1. Field of the Invention
The present invention relates to a terminal of a mobile communication system, and more particularly to a power amplifier for a transmitter of a mobile communication terminal.
2. Description of the Related Art
A transmitter of a mobile communication terminal usually includes a power amplifier. CMOS (Complementary Metal-Oxide Semiconductor) technology has enabled on-chip integration of the power amplifier, which has made a contribution to minimize BOM (Build on Materials) and areas for mounting devices (i.e., footprints). Also, the on-chip integration technology of the power amplifier has allowed not only for the elimination of the requirement that the substrates and the common mode noises thereon by fully differential (pseudo) structure thereof, but also to reduce the PAPR (Peak-to-Average Power Ratio), for example, voltage of 3 dB and power of 6 dB. Besides, through the on-chip integration technology, it is still possible to minimize non-linearity caused by the influence of the PAPR when the voltage levels are equal (i.e. equal consumption power).
In addition to the on-chip integration technology, an on-chip balun transformer for use as an antenna interface may be applied to the power amplifiers. This allows to further reduce losses as compared with the discrete devices usually mounted in the outside.
It has been difficult to realize the conventional power amplifiers by the COMS technology because the power amplifiers based on the CMOS technology require relatively larger currents for obtaining a desired gain. In recent years, however, as the ωC in the impedance has greatly increased in light of the reduced channel lengths in the CMOS, the use of the power amplifiers of the CMOS type is increasing in fields of wireless LAN and mobile communication (for high frequency).
Other types of power amplifiers in the prior art consist of power amplifiers of GaAs (MESFET) and InPGaAs types which require relatively little current as compared with those of CMOS type but are expensive due to the lower yields thereof. It is also difficult to realize the amplifiers of the GaAs type and the InPGaAs type in the form of Si system-on-chip because they are manufactured by using none-silicon materials. It is still another problem that differential structures used in the GaAs and the InPGaAs employ discrete baluns outside of the structure, which increase the footprints and loss.
As described above, the conventional power amplifiers of the GaAs type and the InPGaAs type are expensive and have a low yield. In the conventional power amplifiers of the CMOS type, it is difficult to obtain the necessary gain for a relevant current due to the low trans-conductance thereof. Furthermore, the conventional power amplifiers generate considerable spurs (e.g., harmonics and IM—Intermodulation) although they are square-low devices. Also, the conventional power amplifiers have a single-ended (antenna interface) structure, so that the conventional power amplifiers do not have sufficient voltage swings based on a fixed power supply voltage and thus fail to provide effective electrical power to the final output loads.
According to reduction in the channel interval in CMOS technology, an available frequency range in CMOS has gradually increased so that it has become possible to supply sufficient electrical power (not more than 0.35 um thereof), and to design and realize the power amplifiers in the form of on-chip through the CMOS process.
FIG. 1 is a schematic diagram of the conventional power amplifier using the CMOS technology according to the prior art. The conventional power amplifier of the CMOS type includes at least one differential amplifier 20 to 23 of the same type. The differential amplifiers 20 to 23 are similar to the well-known existing differential amplifiers. The conventional power amplifier includes transformers 10 to 13 which are connected to the outputs of the amplifier and correspond to the differential amplifiers 20 to 23. The conventional power amplifier of the CMOS type, which generates a desired output level by stacking the voltages charged in capacitors 30 to 33 through the transformers 10 to 13, requires at least four differential amplifiers 20 to 23. The conventional power amplifier converts each output (current) of the differential amplifiers into a voltage through the transformers 10 to 13 (differential to single ended) instead of the typical cascading stages, and then adds it to a voltage stacked in an inductor which is a part of each transformer.
However, the output of the conventional power amplifier of the CMOS type is insufficient to satisfy the design specifications required in the mobile communication devices. Further, in a state where the power amplifier of the single-in/out type prevails in the art, the power amplifier of the differential type which can be directly adapted to the outputs of an input stage transceiver (in the form of IC) is also problematic in that the voltage swing may half-reduce and consequently the saturation voltage may half-reduce.